An image displayed on a cathode ray tube (CRT) may suffer from imperfections or distortions such as defocusing or nonlinearity that is incident to the scanning of the beam on the CRT. Such imperfections or distortions occur because the distance from the electron gun of the CRT to the faceplate varies markedly as the beam is deflected, for example, in the horizontal direction. Reducing the defocusing that occurs as the beam is deflected in the horizontal direction, for example, may be obtained by developing a dynamic focus voltage having a parabolic voltage component at the horizontal rate and applying the dynamic focus voltage to a focus electrode of the CRT for dynamically varying the focus voltage. It is known to derive the parabolic voltage component at the horizontal rate from an S-correction voltage developed in an S-shaping capacitor of a horizontal deflection output stage.
The CRT that employs dynamic focus may have internal wiring that places the dynamic focus voltage close to, for example, the blue electron gun. In normal operation, the proximity to the blue electron gun may not cause any problem. However, when a low current bias measurement is made in an automatic kine bias (AKB) circuit, during several video line times that immediately follow vertical retrace, referred to as the AKB measurement interval, stray coupling of the horizontal component of the dynamic focus voltage may introduce an error in the biasing of the cathode electrode of the blue electron gun. As a result, the bias of the blue electron gun may not track the bias of the green and red electron guns. This may lead to unacceptable background color temperature changes.
It may be desirable to remove the horizontal dynamic focus voltage component from the focus electrode, during the AKB measurement interval. Thereby, the undesirable coupling to the focus electrode is, advantageously, eliminated. It may also be desirable to prevent a significant transient of the focus voltage, after the end of the AKB measurement interval. Such transient could occur when the horizontal dynamic focus voltage is restored, after the AKB measurement interval.
In carrying out an inventive feature, during vertical retrace and during the AKB measurement interval, the focus voltage amplifier prevents coupling of energy from the dynamic focus circuit to the AKB circuit. Instead, a constant current is fed to a load resistor to develop an output voltage of the amplifier that is approximately equal to the positive peak of the summed parabolic voltage components, during the AKB measurement interval. Advantageously, this is done to prevent the aforementioned transient focus voltage error of several hundred volts when vertical scanning resumes at the left end of the first visible horizontal line following the AKB measurement interval.
A video imaging apparatus, embodying an aspect of the invention, includes an amplifier having a first input responsive to a focus voltage correction input signal for generating at an output of the amplifier a dynamic focus voltage component of a focus voltage that is coupled to a focus electrode. A first semiconductor switch is responsive to a periodic control signal and coupled to a signal path of the focus voltage correction input signal for disabling the dynamic focus voltage component, during an automatic kine bias measurement interval of a deflection cycle. A second semiconductor switch is responsive to a periodic control signal and coupled to the amplifier for applying a second input signal to a second input of the amplifier to control the focus voltage, during the automatic kine bias measurement interval. Outside the automatic kine bias measurement interval, the second input signal is decoupled from the amplifier.